Serotonin (5-HT) plays critical roles in enteric physiology and pathophysiology. Gastrointestinal (GI) motility is abnormal, in humans as well as in animals, when mucosal expression of the serotonin transporter (SERT), the molecule mainly responsible for 5-HT inactivation, is deficient. Enteric serotonergic signaling is inadequately understood despite its importance and clinical significance. Two isoforms of tryptophan hydroxylase (TpH-1 and TpH-2) have recently been demonstrated. Although both are expressed in the gut, neither the identities of the enteric cells that express TpH-1 and/or TpH-2, nor how either isoform is regulated in the bowel are known. We thus propose to identify cells in the gut that express TpH-1 and/or TpH-2 and to utilize transgenic mice lacking SERT to determine how increased 5-HT availability affects the expression of TpH-1 and TpH-2. Transcripts encoding 5-HT1B/1D, 5-ht1f, 5-ht5, 5-ht6, and 5-HT7 receptors have been detected in the gut but their enteric actions are unknown; moreover, important responses of enteric neurons to 5-HT have not been attributed to a classified 5-HT receptor. The gut thus contains 5-HT receptors that are "looking for a function" and functions of 5-HT that are "looking for a receptor." Some of these functions are mediated by a receptor activity called 5-HT1p, which has been characterized pharmacologically and with respect to transduction coupling, but has resisted cloning. Conceivably, the properties of 5-HT receptors expressed in enteric neurons may be different from those of the same receptors expressed in heterologous cells of the CNS, where most have been characterized. 5-HT receptors may also acquire novel properties in enteric neurons by forming homo- and/or hetero-oligomers. We now propose to identify cells of the bowel that express receptors that are "looking for a function" (5-HT1B/1D, 5-ht1f, 5-ht5, 5-ht6, and 5-HT7) and to identify enteric neuronal responses these receptors mediate. We will also determine whether 5-HT1B or other 5-HT receptors in enteric neurons form homo- or hetero-oligomers with 5-HT or a D2 dopamine receptor and, if they do, whether homo- or hetero-oligomers acquire 5-HT1p-like activity. Finally, intrinsic enteric dopaminergic neurons have recently been identified by not functionally characterized. We now propose to identify neurons that express DA receptors, to determine whether they receive a dopaminergic innervation, and how these neurons respond to DA and to DA receptor subtype-selective agonists and antagonists. [unreadable] [unreadable]